Molds for wafer scale molding of protective caps

ABSTRACT

Two molds ( 102, 104 ) are provided formed of semiconductor material. The molds ( 102, 104 ) have substantially planar working faces ( 108, 110 ) into which recesses ( 106, 112 ) are formed. In use the molds ( 102, 104 ) are pressed together with the working faces ( 108, 110 ) opposed so the recesses ( 106, 112 ) form mold cavities. The molds ( 102, 104 ) only contact each other in the plane of the working faces ( 108, 110 )

TECHNICAL FIELD

[0001] This invention relates to the molding and application ofprotective caps to microelectronic semiconductor chips on a wafer scaleas opposed to application on an individual chip basis. More particularlythe invention relates to the molds used to form the protective caps.

BACKGROUND ART

[0002] Semiconductor chips are normally packaged in a protective layeror layers to protect the chip and its wire bonds from atmospheric andmechanical damage. Existing packaging systems typically use epoxymolding and thermal curing to create a solid protective layer around thechip. This is normally carried out on individually diced chips bonded tolead frames and so must be done many times for each wafer. Alternativemethods of packaging include hermetically sealed metal or ceramicpackages, and array packages such as ball grid array (BGA) and pin gridarray (PGA) packages. Recently wafer scale packaging (WSP) has startedto be used. This is carried out at the wafer stage before the chips areseparated. The use of molding and curing techniques subjects the waferto both mechanical and thermal stresses. In addition the protective capso formed is a solid piece of material and so cannot be used for MEMSdevices, since the MEMS device would be rendered inoperable by thepolymer material. Existing packaging systems for MEMS devices includethematically sealed packages for individual devices, or use silicon orglass wafer scale packaging, both of which are relatively high costoperation.

DISCLOSURE OF THE INVENTION

[0003] In one broad form the invention provides pair of molds formedsubstantially of silicon or silicon alloy for molding a sheet ofthermoplastic material into an array of microstructures, each of themolds being a substantially planar wafer having a working face and rearface, the molds each having one or more micro fabricated recesses intheir working faces which, when the working faces are placed face toface, define at least one cavity between the two molds, wherein therecesses on the molds are configured such that the molds only contacteach other on the working faces, when the working faces are placed faceto face.

[0004] In plan view the first mold may have at least one first recess inthe respective working face for each cavity and the second mold may haveat least one first groove in the respective working face for eachcavity, such that when the two molds are in use, the at least one firstrecess defines a central portion of the cavity and the at least onegroove defines a perimeter wall portion of the cavity extending from theedges of the central portion.

[0005] Preferably the at least one first recess is, in plan view,generally rectangular.

[0006] One of the molds may have a recess therein having a base and oneor more pillars of mold material extending from the base to the plane ofthe working face.

[0007] One of the molds may have a recess therein having a base andwherein at least part of the base is concave or convex with respect tothe respective mold's working face.

[0008] The other of the molds may have a second recess therein having asecond base and part of the second base may be concave or convex withrespect to the respective mold's working face.

[0009] When two molds are used with concave or convex portions, a lensshaped cavity may be defined.

[0010] The molds are preferably formed of a semiconductor material, suchas silicon.

[0011] The working surfaces of the molds have preferably been preparedand the recesses have been formed using conventional lithography anddeep silicon etching techniques.

[0012] In use, a spacer may be provided between the two molds such thatthe working faces of the molds do not contact each other.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 shows a prior art method of forming protective caps onsemiconductor chips.

[0014]FIG. 2 shows a cross section of a prior art packaging madeaccording to the FIG. 1 method.

[0015]FIG. 3 shows a cross section of a prior art packaging of a MEMSdevice.

[0016]FIG. 4 shows a cross section through a MEMS device packagedaccording to the invention.

[0017]FIG. 5 shows a possible device for forming molded caps;

[0018]FIG. 6 shows method of applying caps formed using the device ofFIG. 5a to a silicon wafer;

[0019]FIG. 7 shows the wafer and caps of FIG. 6 bonded together

[0020]FIG. 8 symbolically shows a method for applying molded caps to asilicon wafer according to the invention;

[0021]FIG. 9 shows the wafer and caps of FIG. 8 bonded together;

[0022]FIG. 10 shows an exploded cross sectional view of a device forforming the protective caps.

[0023]FIG. 11 shows an exploded perspective view of the device of FIG.10.

[0024]FIG. 12 shows a cross sectional view of the device of FIG. 10 atthe commencement of molding.

[0025]FIG. 13 shows the device of FIG. 10 after molding has finished andjust before one side of the mold is released from the other side.

[0026]FIG. 13a shows an expanded view of part of FIG. 13.

[0027]FIG. 14 shows a perspective view of the FIG. 10 devicecorresponding to FIG. 13.

[0028]FIG. 15 shows a cross sectional side view of the device after oneof the molds has been partially removed.

[0029]FIG. 16 shows a cross sectional side view of the device after oneof the molds has been fully removed.

[0030]FIG. 17 shows a cross sectional side view of the device undergoingan etch.

[0031]FIG. 18 shows a cross sectional side view of the device afterundergoing an etch.

[0032]FIG. 19 shows a cross sectional side view of the device at thecommencement of application to a wafer and removal of the second mold.

[0033]FIG. 20 shows a cross sectional side view of a wafer afterapplication of the caps.

[0034]FIG. 21 shows a cross sectional side view of a series of chipsafter singulation of the wafer.

[0035]FIG. 22 shows a cross sectional side view of a wafer with capsapplied to both sides, before singulation of the wafer.

BEST MODE OF CARRYING OUT THE INVENTION

[0036] Referring to FIGS. 1 and 2 there is show a prior art method offorming protective caps on semiconductor wafers on a wafer scale. Asemiconductor wafer 10 is clamped against a mold 12 having cavities 14formed therein and a liquid polymer material 16 is injected into thecavities 14. The polymer material sets to form solid protective caps 18.The wafer is then singulated using a wafer saw. This technique is notapplicable to wafers having MEMS devices formed thereon as the liquidpolymer material will surround the MEMS devices and stop them fromworking.

[0037]FIG. 3 shows the present prior art technique for protecting MEMSdevices. The MEMS chip 20 including the MEMS devices 24, shownsymbolically, is bonded to a silicon wafer 26. This may be carried outat the individual chip stage or at the wafer stage. The wafer 26 istypically etched using a crystallographic anisotropic etch using anetchant such as KOH to form a series of recesses 28 which correspond tothe locations of the MEMS devices. The wafers 26 are carefully alignedwith the MEMS wafer 20 and bonded thereto. While this can be aneffective means of packaging MEMS devices, it is expensive as itrequires an extra silicon (or sometimes glass) wafer, which must beetched to form the cavities.

[0038]FIG. 4 shows a MEMS wafer 30 having surface MEMS 32 formedthereon. A hollow protective cap 34 of thermoplastic material made andbonded to the wafer 30 according to the invention is provided so as toform a mechanical and atmospheric protective barrier for the MEMSdevices. The cap 34 forms a cavity 36 with the wafer to allow the MEMSdevice(s) to operate.

[0039] The use of molded thermoplastic hollow caps offers thepossibility of providing inexpensive packaging. However, conventionaltechniques do not provide the required accuracy and thermal stabilityrequired for micro fabricated devices.

[0040] FIGS. 5 to 7 show a possible technique for packaging asemiconductor wafer 40 having a number of groups 42 of micro fabricateddevices 44, shown symbolically, formed on or in an upper surface 46.

[0041] An array of caps 48 is formed using conventional injectionmolding methods and steel mold tools 50 & 52. The caps are supported ona sprule 54 at the same nominal spacing as the groups 42. Using thismethod will almost invariably lead to misalignment with resultingdestruction of MEMS devices, as shown in FIG. 20. In FIG. 20 the cap 48a has been aligned correctly with its group of MEMS devices 42 a.However the spacing between the caps is greater than the spacing of thegroups so that cap 48 b is not aligned correctly, but does not destroyany of the MEMS devices of its respective group 42 b.

[0042] However, the caps 44 c & d are sufficiently misaligned that theperimeter walls of the caps overlay one or more of the MEMS devices 44,destroying their functionality.

[0043] This misalignment can be the result of a number of actors,including differential thermal expansion of the sprule material comparedto the silicon wafer, non rigidity of the molded components and the lackof machinery designed for accurate alignment and bonding of polymers towafers using these techniques.

[0044] A solution is to use tools which have the same coefficient ofthermal expansion as the wafer, such as silicon and FIGS. 8 & 9symbolically show a technique using a silicon tool 60 to hold an arrayof thermoplastics caps 60 as the caps are bonded to the silicon wafer40. Since the tool 60 is formed of the same material as the wafer 40,changes in temperature will not result in changes in alignment; thespacing of the caps 60 will change by the same amount as the spacing ofthe groups 42 of MEMS devices 44. Thus, when bonded, all of the capswill be correctly aligned, as shown in FIG. 9. Additionally there ismuch experience in working silicon to the required accuracy.

[0045] Forming Caps

[0046] FIGS. 10 to 16 schematically show a first system for creating andapplying hollow protective caps to wafers, preferably semiconductorwafers.

[0047]FIG. 10 shows a molding system 100 for forming the hollowprotective caps shown in FIG. 4 which may be used with MEMS devices orany other micro fabricated device. The molding system 100 includes twosilicon wafers 102 & 104. The upper wafer 102 has been processed usingconventional lithography and deep silicon etching techniques to have aseries of recesses 106 in its lower surface 108. The lower wafer 104 hasbeen similarly processed so that its upper surface 110 has a series ofgrooves 112 which align with edges of the recesses 106. The recesses 106and grooves 112 are sized for the chip size of the wafer to be processedand repeat at centers corresponding to the repeat spacing on the wafer.In the embodiment shown the protective caps are designed for a MEMSinkjet printhead and so are very long relative to their width in planview. The recesses are rectangular, although the ends of the recessesare not shown. The ends of the grooves 112 are not shown but it is to beunderstood that the grooves 112 at each side of each recess are in factone groove which has a rectangular shape in plan view.

[0048] The grooves 112 for adjacent caps define a portion 114 ofmaterial which has not been etched. Similarly adjacent recesses 106define a portion 116 of material which has not been etched. Theseportions of material 114 & 116 align with each other and when the twowafers are pressed together, the two wafers contact each other at theseportions 114 & 116.

[0049] The two surfaces have been etched so that the groove 112 for theperimeter of the cap is all in the lower wafer 104 and the recess 104for the central portion is all in the upper wafer 102.

[0050] It is not essential that the mold wafers only contact on surfaceswhich have not been etched. Nor is it essential that the central portionis defined by a recess in only one mold or that the perimeter walls bedefined by a groove or recess in only one mold. The effective split linebetween the molds may be located at any position desired and need not beplanar. However, planarity of the split line will typically simplifyfabrication of the molds.

[0051] The assembly 100 also includes an upper release or eject wafer118 and a lower release or eject wafer 120. These upper and lowerrelease wafers are silicon wafers which have been processed utilizingconventional lithography and deep silicon etching techniques to have aseries of release pins 122 and 124 respectively. The upper and lowermold wafers 102 & 104 are formed with corresponding holes 126 & 128respectively which receive the pins 122 & 124. The upper holes 126 arelocated generally toward the center or axis of each recess 106 whilstthe lower holes 128 are located in the grooves 112. However the locationof the holes 126 and 128 is not especially critical and they may beplaced as required for ejection of the molded caps.

[0052] The release pins 122 & 124 have a length greater than the depthof the corresponding holes. When the free ends of the pins 122 alignwith the inner ends of the holes 126, there is a gap 130 between theupper mold wafer 102 and the upper release wafer 118. In this embodimentthe length of the lower pins 124 is the same as the thickness of thelower mold wafer 104. However the length of the pins 124 may be greaterthan the thickness of the wafer or it may be less. When the length ofthe pins 124 is less than the maximum thickness of the lower wafer 104it needs to be greater than the depth of the holes 128, i.e. at leastthe reduced thickness of the wafer 104 at the grooves 112. The lowerwafers 104 and 120 are positioned with the pins 124 part way inserted inthe holes 128 but not extending beyond the holes 128 into the grooves112 and with a gap 132 between the two wafers. The pins 124 preferablyextend to be flush with the ends of the holes so as to form asubstantially planar base to the groove 112.

[0053] The thickness of the mold and release wafers is about 800 micronswhilst the gaps 130 and 132 are of the order of 10 to 100 microns inthickness. However this is not critical.

[0054] The mold tools are preferably etched using cryogenic deep siliconetching rather than Bosch etching as to produce a smoother etch. Boschetching produces scalloping of etched side walls, such as the side wallsof the pin and cap recesses. The scalloping makes the release of themolds from the molded material more difficult. In comparison, using acryogenic etch results in much smother etched walls, with easier moldrelease.

[0055] A sheet 134 of thermoplastic material of about 200 to 500 micronsin thickness is placed between the two wafers 102 & 104 and the assemblyis placed in a conventional wafer bonding machine, such as an EV 501,available from Electronic Visions Group of Sharding, Austria.

[0056] The assembly is mechanically pressed together in the machine butit will be appreciated that the mold wafers may be urged toward eachother to deform the thermoplastic sheet by applying an above ambientpressure to the gaps 130 & 132. Alternatively other means may be used.

[0057] The sheet 134 may be heated by conduction but is preferablyheated by radiation and preferably by using infrared radiation, asindicated by arrows 136 in FIG. 12. A combination of conductive andradiant heating may be used. The mold and release wafers 102 &104 and118 & 120 respectively are formed of silicon, which is substantiallytransparent to infrared light of a wavelength in the range of about 1000nm to about 5000 nm. The material 134 chosen either intrinsicallyabsorbs light within this wavelength range or is doped so as to absorblight within this wavelength range. If the material 134 does notintrinsically absorb within this range, a suitable dopant is “carbonblack” (amorphous carbon particles) which absorbs light at thesewavelengths. Other suitable dopants may be used.

[0058] The sheet 134 is placed between the two mold wafers and exposedto infrared light at a suitable wavelength, as indicated by arrows 136.The infrared radiation is preferably supplied from both sides of thewafers and the sheet 134 to provide symmetrical heating, but this is notessential and the infrared radiation may be supplied from only one side.Because the silicon wafers are transparent to the infrared radiation,the infrared radiation passes through the wafers and is absorbed by thesheet 134. After heating to a suitable temperature the mold wafers maythen be urged together to deform the sheet 134. The wafers may bepressed together whilst the sheet 134 is being heated rather thanwaiting for the sheet 134 to be fully heated, particularly if conductiveheating is being used. If a material other then silicon is used heatingof the sheet 134 may be achieved using electromagnetic radiation atother wavelengths to which the material used is substantiallytransparent.

[0059] When processed in a wafer bonding machine the sheet 134 is moldedto the shape of the cavity defined by the recess 106 and the groove 112.The material is also substantially squeezed out of the gap between thetwo portions 114 & 116, as indicated by arrows 142 in FIG. 13a, to forma series of caps 138

[0060] As previously mentioned, the molding wafers 102 & 102 are formedusing conventional lithography and deep silicon etching techniques. Theaccuracy of this process is dependant on the lithography and the resistused. The etch selectivity of silicon versus resist is typically betweenabout 40:1 and about 150:1, requiring a resist thickness for a 500 μmthick etch of between about 15 μm and 4 μm respectively. Using a contactor proximity mask, critical dimensions of around 2 μm can be achieved.Using steppers, electron beam or X-ray lithography the criticaldimensions can be reduced to less than a micron. Thus the material 134may be squeezed out totally from between the portions 114 & 116, totallyseparating the adjacent caps 136. Alternatively a thin layer 140 up toabout 2 microns thick may be left between the portions 114 & 116 betweenadjacent caps 136 due to the variation in position of the relativesurfaces due to manufacturing tolerances.

[0061] It is not essential that the mold wafers or the release wafers bemade of semiconductor materials or that they be processed usingconventional lithography and deep silicon etching methods. Othermaterials and methods may be used if desired. However, the use ofsimilar materials to the semiconductor wafers provides better accuracysince temperature changes have less effect. Also lithography and deepsilicon etching methods are well understood and provide the degree ofaccuracy required. In addition, the one fabrication plant may be usedfor production of both the semiconductor devices and the moldingapparatus.

[0062] It will be appreciated that the two mold wafers 102 & 104 willneed to be shaped so that there is space for the material to move intoas it is squeezed out from between the two wafers.

[0063] After forming of the protective caps 138 it is preferred toremove the lower mold and release wafers 104 & 120 whilst leaving thematerial 134 still attached to the upper mold wafer 102. A vacuum isapplied to the gap 132 between the lower mold and release wafers. Therelease wafers 118 & 120 are mounted in the assembly so as to beimmovable whilst the mold wafers 102 & 104 are movable perpendicular tothe general plane of the wafers. Accordingly, the lower mold wafer 104is drawn downwards to the release wafer 120. The pins 124 of the releasewafer 120 firmly press against the material 134 and so retain thematerial 134 in position and prevent it moving downwards with the lowermold wafer 124. The configuration of the assembly 100 after this stageis shown in FIG. 15.

[0064] The lower release wafer 120 now only contacts the material 134 bypins 124 and so it is now relatively easy to remove the lower releasewafer 120 from contact with the material 134 without dislodging thematerial from the upper mold wafer 102. This is done and the assembly isthen in the configuration shown in FIG. 16, with the material 134exposed for further processing and attachment to a wafer.

[0065] Whilst still attached to the upper mold, the sheet 134 is thensubject to an etch, preferably an oxygen plasma etch, from below, toremove the thin layer 140 of material, as shown in FIG. 17. The etch haslittle effect on the rest of the material due to the significant greaterin thickness of the rest of the material. The etched assembly is shownin FIG. 18.

[0066] The assembly is then placed over a wafer 144 having a number ofchips formed on the wafer. Each chip has a plurality of MEMS devices146. The components are aligned and then placed in a conventional waferbonding machine, such as an EV 501 to bond the caps 138 to the wafer.The array of chips is positioned so that each cap overlays part or allof a chip. The devices are shown symbolically and may be MEMS devices,MOEMS devices, other micro fabricated devices, passive electronicelements or conventional semiconductor devices.

[0067] The assembly is removed from the wafer bonding machine and avacuum is then applied to the upper gap 130 so as to draw the upper moldwafer 102 up toward the upper release wafer 118. Similar to the releaseof the lower mold wafer, the caps 138 are held in place by the pins 122of the upper release wafer. Thus the chance of accidental detachment ofany of the caps from the wafer due to the act of removing the upper moldwafer is reduced, if not totally prevented.

[0068] The wafer 144 is now in a state where each chip is protected by adiscrete cap 138. The wafer can then be singulated into individual die.If the chips are arranged in a regular array, the conventional methodsof wafer singulation—sawing or scribing may be used. However, if theseparation lines between chips are not regular or if the chips are toofragile for sawing or scribing, deep reactive ion etching (DRIE) may beused to singulate the wafers. Although DRIE is much more expensive thanwafer sawing, this is moot if the wafer already required through waferdeep etching, as is the case with an increasing number of MEMS devices.If etching is used, the wafer 144 is next subject to a deep silicon etchin an etching system, such as an Alcatel 601 E or a Surface TechnologySystems Advanced Silicon Etch machine, to separate the wafer 144 intoindividual packages. This etch is carried out at a rate of about 2 to 5microns per minute and may be applied from either the cap side of thewafer or the bottom side of the wafer. The etch is highly anisotropic(directional) so there is relatively little etching of silicon sidewaysof the direction of the etch. If the etch is applied from the caps side,the caps 138 act as masks and only the silicon material between the capsis etched. The etching continues until all the silicon material betweenindividual chips is removed, thereby separating the chips 148 forsubsequent processing. If the etch is applied from below, a separatemask will need to be applied to the bottom surface of the wafer.

[0069] Any silicon exposed to the direction of the deep etch at theseparation stage will be etched away. Thus if the etch is from the top(cap) side any exposed silicon which needs to be retained, such aselectrical bond pads, on the upper surface of the chip should beprotected, such as by a resist, which must be removed prior to wirebonding. An alternative is to apply a mask to the lower surface of thewafer and to deep silicon etch from the rear. Alternatively second capsmay be provided for the lower surface of the wafer, utilizing the samemanufacturing methods as for the upper caps and using the lower caps asmasks for the etch. By providing both upper and lower caps at the waferstage, each chip is substantially completely packaged prior tosingulation.

[0070]FIG. 22 shows a technique for providing protective caps for boththe upper and lower surfaces. The figure shows a wafer 150 upon whichhave been formed a series of MEMS device chips 153 on an upper surface154. Each chip 153 includes one or more MEMS devices 152 and optionallyother micro fabricated elements. A first set of protective caps 156 havebeen formed on the upper surface 154 as per the techniques of theinvention previously described. The bond pads 158 of the individualchips 153 are on the upper surface 154 and are not covered by theprotective caps 156. A second set of protective caps 160 have beenformed on the lower surface 162 of the wafer as per the techniques ofthe invention previously described. The first and second sets ofprotective caps may be applied to the wafer sequentially or may beapplied to the wafer simultaneously. The order of application is notimportant. The second set of caps 160 are located under each chip 153but are larger than the first set 156 and extend under and beyond thebond pads 158.

[0071] The wafer 150 is then subject to a deep silicon etch from thelower surface of the wafer as indicated by arrows 164, rather than fromthe upper surface, to separate the individual chips. The lower caps 160thus act as a mask to the bond pads 158 and because the etching processis very directional, only silicon between the lower caps 160 of theindividual chips is etched away. The bond pads 158 and other exposedparts on the upper surface within the outline of the lower caps aresubstantially unaffected by the etch and so the chips 152 will not bedamaged by the etch.

[0072] It will be appreciated that the provision of the second set ofcaps is only a necessity where a hollow space is required; if a secondset of caps is unnecessary or undesirable, a resist may be coated ontothe lower surface with a grid pattern to leave areas between the chipsexposed for deep etching.

[0073] Throughout the specification, reference is made to semiconductorsand more particularly silicon semiconductors. It is to be understoodthat the invention is not limited to use on semiconductors or siliconbased semiconductors and has application to non semiconductor devicesand to non silicon based semiconductors, such as those based on galliumarsenide semiconductors.

[0074] Whilst the invention has been described with particular referenceto MEMS devices, it is to be understood that the invention is notlimited to MEMS or MOEMS devices and has application to any deviceswhich are or may be bulk fabricated on a wafer.

[0075] It will be apparent to those skilled in the art that many obviousmodifications and variations may be made to the embodiments describedherein without departing from the spirit or scope of the invention.

1. A pair of molds formed substantially of silicon or silicon alloy formolding a sheet of thermoplastic material into an array ofmicrostructures, each of the molds being a substantially planar waferhaving a working face and rear face, the molds each having one or moremicro fabricated recesses in their working faces which, when the workingfaces are placed face to face, define at least one cavity between thetwo molds, wherein the recesses on the molds are configured such thatthe molds only contact each other on the working faces, when the workingfaces are placed face to face.
 2. The pair of molds as claimed in claim1 wherein, in plan view one of the molds has at least one first recessformed in the respective working face for each cavity and the other moldhas at least one first groove formed in the respective working face foreach cavity, and wherein, when the two molds are in use, the at leastone first recess defines a central portion of the cavity and the atleast one first groove defines a perimeter wall portion of the cavityextending from the edges of the central portion.
 3. The pair of molds asclaimed in claim 1 wherein one of the molds has a recess therein havinga base and one or more pillars of mold material extending from the baseto the plane of the respective working face.
 4. The pair of molds asclaimed in claim 1 wherein one of the molds has a recess therein havinga base and wherein at least part of the base is concave or convex withrespect to the working face.
 5. The pair of molds as claimed in claim 1wherein: one of the molds has a first recess therein having a first baseand wherein at least one part of the first base is concave or convexwith respect to the mold's working face; and the other of the molds hasa second recess therein having a second base and wherein at least onepart of the second base is concave or convex with respect to the mold'sworking face; and wherein the first concave or convex parts of the firstand second molds define a lens shaped cavity there between.
 6. The pairof molds as claimed in claim 1, wherein the working surfaces have beenpolished using conventional semiconductor wafer polishing techniques andthe recesses have been formed using conventional semiconductor etchingtechniques.